its beautiful metallic luster for a long time when by itself;
but as soon as it is amalgamated with any other metal it
becomes rapidly dim or oxidized, and in consequence of its
continuous oxidation increases in weight.
I have preserved pure tin for many years without its changing
its silvery luster, while different alloys of this metal which
I have prepared for technical purposes have behaved quite
otherwise.
I have seen in the museum at Cortonne Etrusean inscriptions
upon plates of pure lead which are perfectly preserved to this
day' although they date from very ancient times; on the other
hand, I have found with astonishment in the gallery of Florence
that the so-called "piombi" or leaden medallions of different
popes, in which tin and possibly some arsenic have been mixed
to make them harder and more beautiful, have fallen completely
to white powder, or have changed to their oxides, though they
were wrapped in paper and preserved in drawers.
In the same way I have observed that the alloy which was used
for soldering the copper plates upon the movable roof of the
observatory at Florence has changed rapidly and in places of
contact with the copper plates has gone over into a white
oxide.
I have heard also in England that the iron nails which were
formerly used for fastening the copper plates of the sheathing
of ships were attacked on account of contact, and that the
holes became enlarged until they would slip over the heads of
the nails which held them in position.
It seems to me that this is sufficient to show that the metals
in these cases exert a mutual influence upon each other, and
that to this must be ascribed the cause of the phenomena which
they show by their combination or contact.
After discussing some of the experiments on nerve stimulation
which had been made by Galvani and others, Fabroni argues that
these are principally, if not wholly, due to chemical action,
and that the undoubted electrical phenomena which sometimes
accompany them are not the cause of the muscular contractions.
In discussing the nature of the chemical changes produced in
two metals by their mutual contact, Fabroni says:
'Since the metals have relationships with each other, the
molecules must mutually attract each other as soon as they come
into contact. One can not determine the force of this
attraction, but I believe it is sufficient to weaken their
cohesion so that they become inclined to go into new
combinations and to more easily yield to the influence of the
weakest solvents.'
In order to further show the weakening of cohesion by the
contact of two metals, Fabroni describes the results of some
experiments which he has made. He says:
'In order to assure myself of the truth of my assumptions, I
put into different vessels filled with water:
(1) Separate pieces, for example, of gold in one, silver in
another, copper in the third, likewise tin, lead, etc.
(2) In other similar vessels I put pieces of the same metals in
pairs, a more oxidizable and a less oxidizable metal in each
pair' but separated from each other by strips of glass
(3) Finally, I put in other vessels pairs of different metals
which were placed in immediate contact with each other.
The first two series suffered no marked change, while in the
latter series the more oxidizable metal became visibly covered
with oxide in a few instants after the contact was made.'
Fabroni found that under the above circumstances his oxidizable
metals dissolved in the water, and in some cases salts were
formed which crystallized out. He then compares the metals in
contact with each other in water with the metals on the tongue
when brought into contact, as in Sulzer's experiment, and the
two metals touching each other by which different points on a
nerve were touched to produce the muscular twitchings in
Galvani's experiments, and concludes that the chemical action
upon the metals was the same in each case, and that the other
phenomena observed must have resulted from this chemical
action. It is not strange that when Volta showed later that an
electric current passed between the metals in all of tho above
cases Fabroni should regard the chemical action which he had
previously observed as the cause of this current.
Ten years after the publication of Fabroni's original paper,
Volta wrote a letter to J. C. Delamethrie which was published
in Vol. I of Nicholson's Journal. This letter was written after
the chemical changes in the voltaic cell had received a great
deal of attention by many experimenters, the most prominent of
whom was Davy. To show that Volta's theory as to the source of
the current was not affected by these investigations, a
quotation from this letter is given below.
'You have requested me to give you an account of the
experiments by which I demonstrate, in a convincing manner,
what I have always maintained, namely, that the pretended
agent, or GALVANIC FLUID, is nothing but common electrical
FLUID, and that this fluid is incited and moved by the simple
MUTUAL CONTACT OF DIFFERENT CONDUCTORS, particularly the
metallic; strewing that two metals of different kinds,
connected together, produce already a small quantity of true
electricity, the force and kind of which I have determined;
that the effects of my new apparatus (which might be termed
electromotors), whether consisting of a pile, or in a row of
glasses, which have so much excited the attention of
philosophers, chemists, and physicians; that these so powerful
and marvelous effects are absolutely no more than the sum total
of the effects of a series of several similar metallic couples
or pairs; and that the chemical phenomena themselves, which are
obtained by them, of the decomposition of water and other
liquids, the oxidation of metals, &c., are secondary effects;
effects, I mean, of this electricity, of this continual current
of electrical fluid, which by the above mentioned action of the
connected metals, establishes itself as soon as we form a
communication between the two extremities of the apparatus, by
means of a conducting bow; and when once established, maintains
itself, and continues as long as the circuit remains
interrupted.'[3]
[3] This seems to be a misprint for uninterrupted.
Further along in the same letter Volta reiterates his
conviction that the contact of the two metals furnishes the
true motive power of the current. Thus he says (p. 138):
'As to the rest, the action which excites and gives motion to
the electric fluid does not exert itself, as has been
erroneously thought, at the contact of the wet substance with
the metal, where it exerts so very small an action, that it may
be disregarded in comparison with that which takes place, as
all my experiments prove, at the place of contact of different
metals with each other. Consequently the true element of my
electromotive apparatus, of the pile, of cups, and others that
may be constructed according to the same principles, is the
simple metallic couple, or pair, composed of two different
metals, and not a moist substance applied to a metallic one, or
inclosed between two different metals, as most philosophers
have pretended. The humid strata employed in these complicated
apparatus are applied therefore for no other purpose than to
effect a mutual communication between all the metallic pairs,
each to each, ranged in such a manner as to impel the electric
fluid in one direction, or in order to make them communicate,
so that there may be no action in a direction contrary to the
others.'
At the end of the above letter as published in Nicholson's
Journal, the editor, William Nicholson, comments at length on
Volta's theory of the source of current in the cell and calls
attention to the fact that Davy had already made cells by the
use of a single metal and two different liquids. At the
conclusion of his comments he calls attention to the fact that
Bennett and Cavallo had performed experiments with contact
electrification prior to Volta's experiments, and says in
conclusion, after referring to Bennett,
'This last philosopher, as well as Cavallo, appears to think
that different bodies have different attractions or capacities
for electricity; but the singular hypothesis of electromotion,
or a perpetual current of electricity being produced, by the
contact of two metals is, I apprehend, peculiar to Volta.'
This peculiar theory of Volta's probably never gained many
adherents and was necessarily abandoned as soon as the energy
relations of the current were considered, but the controversy
as to whether the electrical current or the accompanying
chemical changes was the primary phenomenon soon became
transferred to a quite different field, viz., to the origin of
the electrical charges which Bennett had shown resulted from
the contact of different metals. Bennett attempted to account
for the phenomena which he had observed on the hypothesis that
different substances "have a greater or less affinity with the
electric fluid," and Cavallo says:
'I am inclined to suspect that different bodies have different
capacities for holding the electric fluid.'
Volta reaches a similar conclusion after repeating some of
Bennett's experiments. In referring to this decision of Volta
as to the origin of the electric charge in contact
electrification, Ostwald says:
'We stand here at a point where the most prolific error of
Electrochemistry begins, the combating of which has from that
time on occupied almost the greater part of the scientific work
in this field.'
The error, from Ostwald's point of view, lies in the assumption
that the transference of electricity from the one metal to the
other is a primary phenomenon of metallic contact. He, with
many others, including some of the most distinguished
physicists and chemists of the past century, regard the
electrical transference as a secondary phenomenon resulting
from the previous oxidation of one of the metals. Thus Lodge,
in discussing the opposite electrification of plates of zinc
and copper when brought into contact says:
'The effective cause of the whole phenomenon in either case is
the greater affinity of oxygen for zinc rather than copper.'
The apparent conflict of opinion between those who hold that
the different affinities of the metals for oxygen is the cause
of the rearrangement of their electrical charges when brought
into contact and those who hold with Bennett and Cavallo that
the metals in their natural state have different affinities for
the electrical fluid must disappear when we recognize that all
affinity, and consequently the affinity for oxygen, must be an
electrical attraction. If zinc has an affinity for oxygen, it
must be because the zinc is either electropositive or
electronegative to oxygen. If it has a greater affinity for
oxygen than copper has, then the zinc must be either
electropositive or electronegative to copper. This being the
case, and both being conductors, it is not surprising that some
electricity will flow from one to the other when the two metals
are brought into contact.
Those writers who attribute the oxidation theory of contact
electrification to Fabroni apparently overlook the fact that
not oxidation, but the weakening of the cohesion of at least
one of the metals due to their contact, was the primary
phenomenon in Fabroni's theory. When this is remembered, it is
seen that the observations of Bennett and Fabroni, instead of
furnishing arguments for two conflicting theories, actually
serve, as all true scientific observations must, to supplement
each other.
Thus we now know that cohesion or affinity is an electrical
attraction between the atoms or molecules of a body. The only
known methods of changing the electrical attraction between two
bodies whose distances and directions from other bodies remain
constant is by varying the magnitude of their charges or by
changing the specific inductive capacity of the medium between
them. Bennett observed that when two pieces of different metal
in their normal electrical condition are placed in contact,
there is a redistribution of the charges of their surface
atoms. Fabroni observed under the same conditions a change in
the surface cohesion of the two metals.
To the present writer this seems the actual sequence of
phenomena, viz., a redistribution of the charges of the surface
atoms of the metals, a consequent change in surface cohesion
and a resultant oxidation of one of the metals.
ON CERTAIN RESEMBLANCES BETWEEN THE EARTH AND A BUTTERNUT
BY PROFESSOR A, C. LANE
TUFTS COLLEGE
THE drama of the earth's history consists in the struggle
between the forces of uplift and the forces of degradation. The
forces of uplift are mainly the outward expression of the inner
energy and heat of the earth, whether they be the volcano
belching its ashes thousands of meters into the air, or the
earthquake, with the attendant crack or fault in the earth's
crust, leading to a sudden displacement, and sending, far and
wide, a death-dealing shock, or those mountain-building
actions, which, though they may be as gentle and gradual as
might be produced by the breathing of mother earth and the
uplifting of her bosom thereby, nevertheless, end in the huge
folds of our mountain ranges.
Against these, there are always working the forces of
degradation--the slow rotting of weathering caused by the
direct chemical action of the moist atmosphere or the
alternation of hot and cold which crumbles rocks far above the
line where rain never falls. Once the rock is rotten and
decayed, it yields readily to the forces of degradation, which
drag it down--the beating of the rain, the rush of the
avalanche or of the landslide, the tumult of the torrent, the
quieter action of the muddy river in its lower reaches or the
mighty glacier which transfers fine and coarse material alike
toward the sea.
These actions are always going on. Are they always equally
balanced, or are there periods when the forces of elevation are
more active, the forces of degradation not so powerful, as
against other times in which the forces of degradation alone
are at work? If there is inequality in the balance and struggle
of these contending forces, the great periods or acts in the
geologic drama might thus be marked off as Chamberlin suggests.
Newbery, Schuchert and others have pointed out that there seem
to have been great cycles of sedimentation which may be
interpreted as due to the alternate success, first of the
factors of elevation, then of those of degradation.
Suppose, for instance, that there has been an epoch of
elevation, that mountain chains have been lifted far into the
sky and volcanoes have sent their floods of lava forth, and
fault-scarped cliffs run across the landscape and that then,
for a while, the forces of elevation cease their work. Little
by little, the mountains will be worn down to a surface of less
and less relief, approaching a plain as a hyperbola approaches
its asymptote--a surface which W. M. Davis has called
peneplain.
But where will the material thus worn go? Into the sea. Going
into the ocean it will raise the level of the sea slowly but
surely. At present, for every four feet of elevation taken off
the land, there will be something like one foot rise of the
ocean level, and this rise may take only thirty thousand
years--a long time in human history, but not so long in the
history of the earth. All the time, then, that the forces of
the atmosphere are wearing down the surface of the earth to the
sea level the sea is rising and its waves are producing a plain
of marine denudation which rises slowly to meet the peneplain
which is produced by degradation. In the beginning of this
cycle, where the forces of degradation have their own way,
coarse material may be brought down by torrents from the
mountains, and the glaciers, which find their breeding place in
these high elevations, may drag down and deposit huge masses of
boulder clay. But, little by little as the mountains are
lowered, the sediments derived from them will become finer and
finer and glaciers will find fewer and fewer sources.
Not only that, but the growth of seas extending over the
continents will tend to change the climate, we shall have a
moister, more insular climate, we shall have a greater surface
of evaporation, and thus, on the whole, a more equable
temperature throughout the world. We know that, at present, the
extremes of cold and hot are found far within the interior of
the continents. Continental climates are the climates of
extremes, and on the whole extremes are hurtful to life. So
then as the forces of degradation tend to lower the continents
beneath the sea level glaciers and deserts and desert deposits
alike must also disappear. Vegetation will clothe the earth,
and marine life swarm in the shallow seas of the broadening
continental shelf. Under the mantle of vegetation, mechanical
erosion will be less, that is, the breaking up of rocks into
small pieces without any very great change, but the rich soil
will be charged with carbon dioxide, and chemical activity will
still go on. Rivers will still contain carbonates, even though
they carry very little mud, and in the oceans the corals and
similar living forms will deposit the burden of lime brought
into the sea by the rivers. Thus, if forces of degradation have
their own way, in time there will be a gradual change in
dominant character, from coarse sediments to fine, from rocks
which are simply crumbled debris to rocks that are the product
of chemical decay and sorting, so that we have the lime
deposited as limestone in one place and the alumina and silica,
in another. We shall have a change from local deposits, marine
on the edges of large continents, or land deposits, very often
coarse, with fossils few and far between, to rocks in which
marine deposits will spread far over the present land in which
will appear more traces of that life that crowded in the
shallow warm seas which form on the flooded continents. We
shall have a transition from deposits which may be largely
formed on the surface of the continents. lakes, rivers, salt
beds and gypsum beds, due to the drying up of such lakes and
the wind-blown deposits of the steppes, to deposits which are
almost wholly marine.
Now, I need not say (to those who are familiar with geology)
that we have indications of just such alternations in times
passed. There are limestones abounding in fossils, with a
cosmopolitan life very wide spread to be recognized in every
continent, such as used to be known as the Trenton limestone,
the mountain limestone, the chalk. Perhaps every proper system
and period should be marked by such a limestone in the middle.
The time classed as late Permian and Triassic on the other hand
was one of uplift, disturbance, volcanic action and extreme
climates, which gave us the traps of Mt. Tom, the Palisades of
the Hudson, the bold scenery of the Bay of Fundy and the gypsum
and red beds which are generally supposed to be quite largely
formed beneath the air and beds of tillite formed beneath
glaciers. Then in the times succeeding, in many parts of the
world, degrading forces were more effective than uplifting so
that the mountains became lower, and the seas extended farther
over the continents. Then the prevalence of lime sediments was
so great that the "chalk" was thought to be characteristic
everywhere. And about the time the "chalk" the land was reduced
to a peneplain. A similar cycle may be traced from the
Keweenawan rocks to the group of limestones so widespread over
the North American continent and so full of fossils, which to
older geologists and oil drillers have been known, in a broad
way, as Trenton.
All this introduces a question--to which I wish to suggest an
answer--How is it that these cycles came to be? Were the outer
rock crust of the earth perfectly smooth the oceans would cover
it to the depths of thousands of feet and it is only by the
wrinkling of such a crust that any part of it appears above the
ocean. If the earth had a cool thin crust upon a hot fluid
interior, and that thin crust were able to sustain itself
during geologic ages so that the shrinkage should accumulate
within, until finally collapse came, giving an era of uplift,
it is obvious that we could account for such cycles. There is
very clear evidence that the outermost layer of the earth's
crust is but a thin shell like the outer shuck or exocarp of a
butternut, so thin that it is not at all possible that it can
sustain itself for more than a hundred miles or so, or for more
than a very few years at the outside. Hayford's[1]
investigations are the latest that show that the continents
project because, on the whole, they are lighter, they float,
that is, above the level of the oceans because there is a mass
of lighter rock below, like an iceberg in the sea. Here the
likeness between nut and earth fails and it would be more like
the earth if the outer shuck were thicker in certain large
areas. If this extra lightness or "isostatic compensation" is
equally distributed, Hayford finds[2] that the most probable
value of the limiting depth is 70 (113 km.) miles, and
practically certain that it is somewhere between 50 (80 km.)
and 100 (150 km.) miles; if, on the other hand, this
compensation is uniformly distributed through a stratum 10 (16
km.) miles thick at the bottom of the crust so that there is a
bulging of the crust down into a heavier layer below to balance
the projection of the mountains above, as I think much more
likely, then the most probable depth for the bottom of the
outer layer is 37 (60 km.) miles. This layer is much thinner
than the outer layer of the figure and is supposed to yield to
weight placed as, though more slowly than, new thin ice bends
beneath the skater.
[1] The figure of the earth and isostasy from measurements in
the U.S. Dept. of Commerce and Labor, 1909, p. 175.
[2] loc. cit., p. 175.
There are a number of facts which support this so-called theory
of isostasy, according to which the crust of the earth is not
capable of sustaining any very great weight, though it may be
at the outside rigid, but is itself essentially like a flexible
membrane resting on a layer of viscous fluid. However viscous
this fluid may be and rigid to transitory quickly shifting
strains like those produced by the earth's rotation, it does
NOT REMAIN AT REST in a state of strain (at any rate if this
strain passes limits which are relatively quite low). Not only
are, according to Hayford's observations, the inequalities of
the North American continent compensated for by lighter
material below, so that the plumb- bob deflections are only one
twentieth what they would be if they rested upon a rigid
substratum of uniform density, but other facts that lead to the
same conclusion are the apparent tendency of areas of
sedimentation to slowly settle under their load, the apparent
settling of the Great Lake region under a load of ice and
springing up again since the removal of the ice. But if the
theory of isostasy is true, one would at first say that there
could be no great accumulation through a geologic period of
stresses which would finally yield in the shape of folded
mountain ranges. It has, in fact, been suggested that mountain
ranges have been slowly folded and lifted as the stress which
produced them accumulated and this would seem to be true if one
considers only the outer crust, but on the other hand, as we
have pointed out, there are indications in the history of the
earth of periods of relative quiescence followed by periods of
relatively considerable disturbance.
How can these two theories be reconciled in accordance with
what we know of the laws of physics and chemistry and those of
the earth's interior? It seems to me they can by making
suppositions which are perfectly natural regarding the state of
the earth's interior.